Not exactly sternum, but I ended up with badly bruised ribs on one side of my body
(the upper ones near my armpit and boob
) after a body-surfing wipeout a few years ago. Four to six weeks to completely heal sounds about right. In my experience there were only about 2 weeks when I really couldn't train properly, but it took another 2 weeks before I was completely pain-free for most swim/bike/run workouts without popping about six Advil a day plus some topical painkillers and ice, and still another two weeks before I could sleep comfortably on that side without a couple of Advil before bed.
Prior to the wipeout, I had entered an Olympic tri two weeks later, and managed to get in some easy workouts in each discipline during the week before the race, but swim and bike were pretty difficult--I could only swim in my shorty wetsuit
(I think the compression helped
), and couldn't do flip turns, sprints, or any strokes but freestyle. Biking on the trainer was much easier than on the road--for some reason getting on and off were painful. Forget about getting into aero position. In retrospect, I should have just ridden sitting up on the trainer. Running was really painful for the first week and part of the second, then it seemed to be fine. I did the race; the swim was a bit sub-par as it hurt to pull strongly on that side, but little actual pain as I held back; the bike also a bit sub-par, I think due to my difficulty staying in aero; the run was fine. What really hurt were transitions--bending over to put on shoes and the like.
I would have scratched from the race, but it was the only Oly I was able to do that year owing to my work schedule and coursework, and the only race my family could attend. If not for the race, I think I would have just laid off running
(the impact probably slowed healing, and I was running too slowly to get much training benefit anyway
), swimming, and road biking, and just ridden the trainer in whatever comfortable position I could find. Unless there's a compelling reason not to, just let pain be your guide. Do what doesn't hurt and return to normal training slowly as it becomes possible without pain. I know I thought 4-6 weeks was ridiculous to heal from simple bruising, but it really did take that long.